Structural stability and carbohydrate contents of an ultisol under different management systems

An understanding of the dynamics of soil carbohydrate pools is necessary for assessing the impact of organic residue management in organic matter build up and structural stability in tropical ecosystems. The objectives of this study were to evaluate temporal changes in aggregate stability and cold M ater-soluble, hot water-soluble and acid-soluble carbohydrate fractions of a sandy soil under different organic residue management practices. The soil is an Nkpologu sandy clay loam (fine-loamy, kaolinitic, isohyperthermic, typic kandiustult) at Nsukka in southeastern Nigeria. In July 1995, it was incorporated with complete fertiliser (N:P:K = 12:12:17 at 480 kg/ha) (F); lice mill wastes: (RW, 10 t/ha), RW + F; poultry manure (PM, 10 t/ ha) and RW (5 t/ha) + PM (5 t/ha) up to the 0-20 cm depth. A control, tilled up to the 0-20 cm depth, was also included. Surface soil samples (0-20 cm), collected at 3, 6 and 12 months after residue applications were used to measure changes in aggregate stability by mean weight diameter (MWD), total OC and carbohydrate pools. In all treatments MWD increased whereas the concentrations of acid-soluble, hot water-soluble and cold water-soluble carbohydrates decreased with sampling time. Also irrespective of the type of amendment, the carbohydrate concentrations at each sampling period varied in the order, acid-soluble > hotwater-soluble > coldwater-soluble. Aggregate stability correlated very poorly with all the carbohydrate fractions and OC. The correlation coefficient values were rather low and did not mean much in the physical interpretation of these results. This shows that these carbohydrate pools were not very effective in stabilizing the soil aggregates. (C) 2001 Elsevier Science B.V. All rights reserved.